Place butter in a large pot and melt it over medium-high heat. Add onion, celery, carrot, potatoes, and squash and saute for 5 minutes, or until lightly browned in color. Add chicken stock enough to cover vegetables and bring to a boil. Reduce heat to low, cover with a lid, and let simmer for 40 minutes or until all vegetables are soft.

Transfer the mixture to a blender and puree until smooth texture is reached. Return the soup to pot, and mix in any remaining stock to attain desired consistency. Add salt and pepper to taste.